The body structure of modern vehicles is developed by assembling and permanently attaching a plurality of generally stamped parts. In one body construction, variously described as unibody, monocoque, or body-frame-integral (BFI), these stamped parts may cooperate in conveying to the vehicle the desired structural rigidity and strength. Alternatively, the body may be mounted to a separate frame structure.
In many instances the stamped body parts will be assembled from an outer body panel, visible to an outside observer and conveying an aesthetically-pleasing appearance and an inner panel, frequently hidden from view by subsequent attachment of plastic or fabric trim. The combination of inner and outer panels positioned and held in spaced apart configuration, creates an efficient structural entity. When additional strength is required the inner and outer panels may be supplemented with a third body stamping, a reinforcement often employed in selected locations only, to provide selective structural enhancement. Two major components of the vehicle body structure are the body side members. These body structures substantially span the length of the passenger compartment and include openings for doors.
The body side structure typically extends above the door window openings and includes a rail, formed as a closed box structure supported on a plurality of pillar or post-like elements which define and bound the door openings. The rail structure may include a flange, suited both for ease of permanently attaching the body inner, outer and reinforcement panels as well as providing a mounting surface for the vehicle roof. The mounting flange is frequently positioned below the top of the body side and interior to the body outer panel so that it is at least partially concealed from anyone observing the outside of the vehicle.
It will be appreciated that vehicles may be viewed as comprising a passenger compartment, and engine compartment and a luggage compartment, which may, in minivans, station wagons and the like, form a part of the passenger compartment.
The body sides are joined to the floor pan to close out the bottom of the passenger compartment and, in the front, to a firewall, a stamped panel which extends the width of the vehicle and separates the passenger compartment from the engine compartment. The rear of the passenger compartment may be terminated and separated from the vehicle trunk in a sedan or coupe by a stamped panel extending the width of the vehicle or by a liftgate in vehicles such as hatchbacks, SUVs or minivans.
The passenger compartment is fully closed out by addition of a roof. Typically a roof resembles an inverted shallow pan, with generally vertical walls and typically curved in at least one direction. The walls of the shallow pan terminate in a generally horizontal flange of substantially-uniform width, extending around the periphery of the roof and shaped to generally conform to the flanges of the body sides. The roof flanges will therefore be supported by the mounting flanges formed in the body sides and may be attached to the body sides at these flanges. The roof will also be attached to and supported by roof headers and bows, closed-section or channel-shaped structures secured to the body sides and extending across the width of the vehicle. These roof headers and bows may also be shaped to cooperatively interact with the pillar structure of the body sides to accept and retain the windshield and rear glass.
Vehicle roofs are commonly fabricated as a stamped sheet metal outer panel, attached to the vehicle body on all sides or to the reinforcing rib or channel structures comprising the roof headers/bows. An inner panel is not normally employed and an attractive interior appearance is provided by installation of a fabric-covered and insulated headliner. If a steel roof is to be attached to a steel body, a series of resistance spot welds is typically used to attach roof and body.
More recently, with interest in reducing vehicle mass, consideration has been given to employing aluminum alloy stampings for vehicle roofs. Because steel and aluminum may not be spot welded together, the aluminum alloy roof is alternatively attached to the steel body using rivets, usually supplemented by adhesive. This approach, though appealing from a vehicle mass-reduction viewpoint, raises issues due to the significantly different coefficients of thermal expansion of aluminum and steel—about 23×10−6 K−1 for aluminum and about 11×10−6 K−1 for steel.
Because the steel and aluminum are permanently joined together by the rivets, this difference in thermal expansion of steel and aluminum will develop stresses in the aluminum and steel whenever the vehicle body temperature differs from the temperature at which the joint was made. The highest temperature experienced by the vehicle body is during manufacture, when the assembled body is painted. Automotive paint consists of a number of layers, applied separately and then cured at elevated temperature. The paint is cured by passing the painted body through one or more paint bake ovens to raise the body temperature to about 180-200° C. and maintain it at that temperature for at least 20 minutes. This temperature excursion may be sufficient to initiate plastic deformation in the aluminum roof. Since plastic deformation is not reversed on cooling, any such deformation may result in an appearance feature such as a crease or buckle in the roof which would be unacceptable to the customer.
There is therefore a need for an improved roof design better suited to resist plastic deformation and the resulting permanent deformation of an aluminum alloy roof attached to a steel body structure during temperature excursions such as those typical of paint bake cycles.